The sex difference in the basal metabolic rate.

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The existence of a sex difference in the basal oxygen consumption is a matter of considerable importance, both of itself and in its broader implications. Since boys have a higher oxygen consumption under basal conditions, beginning even before the eighth year (6, 7), separate norms for boys and girls obviously are necessary. The fact that the basal metabolic rate of the adult male appears to be so much higher than in the female bears upon sex differences in behavior, and in mortality and morbidity as well. Pearl, among others, has attributed the shorter life span of the male to his higher level of energy metabolism, or as Pearl termed it to his faster rate of living. But before accepting a causal relationship between the higher metabolism and the shortened life span of the male, it is well to be sure that the male does have a higher metabolic rate, after due corrections for body size and body composition. It is noteworthy that the large sex difference in oxygen consumption, observable by the early teens, lessens when correction is made for differences in body size, or in the extent of body surface. Even so, there is a residual difference in oxygen consumption which increases after the first decade, but stabilizes before the time of legal majority (6). This residual difference is of especial interest since it parallels changes in body composition. Briefly stated, boys and girls are reasonably comparable in the proportions of fat and muscle until about the iith year (io). Thereafter boys increase markedly in the proportion of muscle, while girls add a higher proportion of fat (9, io). In the adult state this difference continues to be true (x, 12) and may even become quite exaggerated (5)Now, the lean body mass, or more particularly the amount of skeletal muscle, appears to be the single largest determinant of the basal oxygen consumption (4, 8), with correlations between the two variables from 0.8 to 0.9. Thus, males and females or boys and girls of equivalent body weight may still differ considerably in the amount of muscle present, and therefore in basal oxygen consumption. Under these circumstances older boys and